Using dynamic light scattering in heterodyne mode, we measure velocity profiles in two complex fluids known to exhibit stress plateau behaviour under shear: a wormlike micelle solution and a lamellar phase. In both cases, our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap. We point out that the position of the interface between the two structures is stable at a fixed local shear stress sigma*.